RFR: 8325808: GenShen: Move generational mode code out of shFullGC.cpp [v2]

[William Kemper]

> This change reduces the differences from the upstream branch by moving large chunks of generational mode code into separate files.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Improve names and comments

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/398/files
  - new: https://git.openjdk.org/shenandoah/pull/398/files/850b011a..cfed36ea

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=398&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=398&range=00-01

  Stats: 53 lines in 6 files changed: 27 ins; 7 del; 19 mod
  Patch: https://git.openjdk.org/shenandoah/pull/398.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/398/head:pull/398

PR: https://git.openjdk.org/shenandoah/pull/398